The rumors of Dean's negative campaigning are false. While I'd like to say this is due to moral reasons, there is a far less noble reason for this restraint. Dean's campaign strategy in NH is based entirely on positive voter contact, IDing supporters, recruiting volunteers and getting out the vote. They do not engage in negative campaigning not because they object on moral grounds, but because they feel it is a less efficient use of their time.
Passing out flyers to passers by is a poor campaign strategy. For starters, effective campaigning means targeting likely voters. Half of those passers by will be Republicans, the remainder may not vote in the primary, may have irreversably made up their mind, or otherwise are unlikely to be swayed by the flyers. Furthermore, for every voter won over, another will be outraged that we would engage in such an attack, and while they may be convinced not to vote for Clark, they sure won't vote Dean either.
Contacting voters is a science. By this stage in the game, a winning campaign has a database of likely Democratic voters all classified as supporters, likely supporters, undecideds or in another camp. Given such a database and the ability it gives to directly target Clark supporters through phone calls and canvassing, the Dean campaign would never authorize its volunteers to pass out anti-Clark flyers. Again, this decision would be made simply because Dean For America would rather have those volunteers phone banking and canvassing; it is a more beneficial use of their time.
This all said, it is not impossible that UNAUTHORIZED volunteers printed their own flyers and passed them out WITHOUT THE KNOWLEDGE OF THE DEAN CAMPAIGN. It is also conceivable that another campaign decided to engage in this tactic to undermine Dean. What is certain, however, is that absolutely no official Dean staffer authorized this attack, and if they did, they'd be fired.